Welded Joints Complete

7/13/2019 Welded Joints Complete

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Mechanical Engineering Dept. CEME NUST 1

Ch-4: Design of Welded Joints


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Design of Welded Joints

Welded Joint is a permanent joint which is obtained by the fusionof the edges

of the two parts to be joined together, with or without the application of pressureand a filler material

Heat required for the fusion of the material may be obtained by burning of Gas

(in case of Gas Welding) or by an Electric Arc (in case of Electric Arc Welding)

Advantages and Disadvantages of Welded Joints over Riveted Joints

Advantages

Welded Structures are usually Lighterthan riveted structures (Gussets or other

connecting components are not used)provide maximum Efficiency(up to 100%), not possible in case of riveted joints

Alterationsand Additionscan be easily made in the existing structures

smooth in Appearance, therefore looks pleasing


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Advantages--contd--

Welded Joint has a great Strength, usually has the strength of the parent metal

itself

members are of such a shape (i .e. Circular Steel Pipes) that they afford difficulty

for riveting. But they can be easily welded

Welding provides very Rigid Joints

It is possible to weld any part of a structure at any point. But riveting requires

enough clearance

Process of welding takes less time than the riveting


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Disadvantages

Uneven Heating and Cooling during fabrication, therefore the members may get

distorted or additional stresses may develop

Requires a Highly Skilled Labor and supervision

Inspection of welding work is more difficult than riveting work

No provision is kept for expansion and contraction in the frame there is a

possibility of cracks developing in it


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Types of Welded Joints

Lap Joint or the Fillet Joint is obtained by overlapping the plates and then

welding the edges of the plates

Lap Joint

o Cross-sectionof the fillet is approximately Triangular

Single Transverse Fillet Double Transverse Fillet Parallel Fillet Joints


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Butt Joint

Butt Joint is obtained by placing the plates edge to edge

If the Plate Thickness is 5 mm to 12.5 mm, the edges should be beveled to V or

U-groove on both sides

plate edges do not require Bevelingif the thickness of plate is less than 5 mm


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Other Joints

Other type of Welded Joints are Corner Joint, Edge Joint and T-joint

Main considerations involved in the selection of weld type are:

o Shapeof the welded component required

o Thicknessof the plates to be welded

o Directionof the forces applied


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Basic Weld Symbols


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Elements of a Weld Symbols

1. Reference line

2. Arrow

3. Basic weld symbols

4. Dimensions and other data

5. Supplementary symbols6. Finish symbols

7. Tail

8. Specification, process or other references


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Representation of welding symbols

Fillet-weld each side of Tee-convex contour

Single V-butt weld machining finish

Double V- butt weld

Plug weld - 30Groove angle- flush contour


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Further details on Types of Welding Symbols:

Chap-9 (page: 478), Book: ShigleysMechanical Engineering Design, 9th ed.


Representation of welding symbols

Staggered Intermittent Fillet Welds

Circle on the weld symbol welding is to go allaround

o welds are intermittent and staggered 40 mm

along on 100-mm centers


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Strength of Transverse Fillet Welded Joints

Transverse Fillet welds are designed for Tensile Strength

Single Transverse Fillet Double Transverse Fillet

Assumption: section of fillet is a Right Angled

Triangle ABC with hypotenuse AC making equal

angles with other two sides ABand BC

Leg Or Size Of The Weld: Length of each side (ABor

BC)

Throat Thickness: Perpendicular distance of the

hypotenuse from the intersection of legs (i.e. BD)


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Strength of Transverse Fillet Welded Joints

t= Throat thickness (BD)

s= Leg or size of weld= Thickness of plate

l= Length of weld

Throat Thickness = t = s sin 45= 0.707 s

Minimum area of the weld or throat area= A= Throat thickness Length of weld

Minimum Area of the weld is taken because the

stress is maximum at the minimum area

= t l = 0.707 s l

P = Throat area Allowable tensile stress = 0.707 s l t

Tensile Strength of the joint for Single Fillet Weld:

Tensile Strength of the joint for Double Fillet Weld:

P = 20.707 s l t = 0.707 s l t

weld is weaker than the plate due to slag and blow holes, therefore the weld is given aReinforcementwhich may be taken as 10%of the Plate Thickness


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Strength of Parallel Fillet Welded Joints

Parallel Fillet Welded Joints are designed for Shear Strength

Double Parallel Fillet Weld Combination of transverse

and parallel fillet weld

= Allowable Shear Stress for the weld metal

P = Throat area Allowable Shear Stress = 0.707 s l

Shear Strength of the joint for Single Parallel Fillet Weld:

P = Throat area Allowable Shear Stress = 20.707 s l = 1.414 s l

Shear Strength of the joint for Double Parallel Fillet Weld:

For Combination of Single Transverse and Double Parallel Fillet Welds:

P =0.707s l1t+ 1.414 s l2


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Polar Moment Of Inertia and

section modulus of welds


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Special Cases of Fillet Welded Joints

1. Circular fillet weld subjected to torsion

d= Diameter of rod,

T= Torque acting on the rod,

s= Size (or leg) of weld,

t= Throat thickness,

J = Polar moment of inertia of the weld section =

Shear Stress of the material is:

maximum shear occurs on the throat of weld which is inclined at 45to the horizontal

plane

Length of throat: t = s sin 45= 0.707 s


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Special Cases of Fillet Welded Joints

2. Circular fillet weld subjected to Bending Moment

d= Diameter of rod,

M= Bending moment acting on the rod,

s= Size (or leg) of weld,

t= Throat thickness,

Z= Section modulus of the weld section

Bending Stress:

maximum shear occurs on the throat of weld which is inclined at 45to the horizontal

plane

Length of throat: t = s sin 45= 0.707 s


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Strength of Butt Joints

Butt Joints are designed for tension or compression

In case of butt joint, length of leg or size of weld is equal to throat thickness

which is equal to thickness of plates

Tensile Strength of the butt joint (single-V or square butt joint)

P = t l t

Tensile Strength for Double-v Butt Joint

P = (t1+ t2) l t

t1= Throat thickness at the top

t2= Throat thickness at the bottom


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Figure shows a horizontal steel bar of thickness hloaded in steady tension and

welded to a vertical support. Find the load Fthat will cause an allowable shearstress, allow, in the throats of the welds.

Example 2.1



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Eccentrically Loaded Welded Joints

Eccentric Load may be imposed on welded joints in many ways

induced stresses are combined depending upon the nature of stresses

When the shear and bending stresses are simultaneously present in a joint, then

maximum stresses are as follows

Maximum Normal Stress

Maximum Shear Stress

b= Bending stress

= Shear stress

a T-joint fixed at one end and subjected to an eccentric

load Pat a distance e

o joint will be subjected to the following two types of stresses

1. Direct shear stress due to the shear force P acting at the

welds

2. Bending stress due to the bending moment P e.

T-joint fixed at one end and subjected to eccentric load


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Eccentrically Loaded Welded Joints

T-joint fixed at one end and subjected to eccentric load contd--

A = Throat thickness Length of weld

= t l 2 = 2 t l ... (For Double Fillet Weld)

= 2 0.707 s l = 1.414 s l ... (t = s cos 45= 0.707 s)

Shear Stress in the weld (assuming uniformly distributed)

Section Modulus of the weld metal through the throat

...(For both sides weld)

Bending moment, M = P e

Bending Stress


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Stresses in Welded Joints in Torsion

A Cantileverof length l welded to a column by two Fillet Welds

Eccentric Load F can be replaced by a

Shearing Force V and a Moment M

o Shear Force produces a Primary Shear /in

the welds of magnitude:

Ais the Throat Area of all the welds

o Moment at the support produces Secondary

Shearor Torsionof the welds:

r= Distance from the Centroidof Weld Group to the point in the weld of interest

J= Second Polar Moment of Area of Weld Group about the Centroid

//is proportional to its distance from the center of twist (r), (//)max

will occurat

the corners of the weld

Primary Shear /is always directed parallel to P

Secondary Shear Stress //can be added vectorially to the Primary Shear Stress / to

determine the Maximum Shear Stress max


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Two Welds in a Group

Rectangles represent the Throat

Areas of the welds

Throat Thickness of Weld-1 = t1= 0.707h1

Throat Thickness of Weld-2 = t2= 0.707h2

h1and h2are the respective Weld Sizes

Throat Area of both welds together

A = A 1+ A 2=t1d + t2b

Second Moment of Area of Weld-1through

G1about x-axisis:

Second Moment of Area of Weld-1through G1about y-axisis:

Second Polar Moment of Area of Weld-1 about its own centroid:


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Two Welds in a Group--contd--Second Polar Moment of Area of Weld-2

about its own centroid:

CentroidG

of the Weld Group is located at

distances r1and r2from G1and G2to G

using the Parallel-axis Theorem, Second Polar Moment of Area of the Weld Group

This is to be used in Torsion Eq.

In a Reverse Procedure, Weld Size can be found for which the Allowable Shear

Stressis given


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Two Welds in a Group--contd--

Setting the weld thicknesses t1and

t2 to Unity leads to the idea of

treating each fillet weld as a Line

Resulting Second Moment of Area is then a Unit Secon d Polar Moment of Area

Since Throat Width of a fillet weld is 0.707h, the relationship between Jand Juis:


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Torsional Properties of Fillet Welds


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A steel bar of thickness h, to be used as a beam, is welded to a vertical support as

shown in the figure. Find the safe bending force Fif the allowable shear stress in

the welds is 140 MPa

Example 2.2



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A 50-kN load is transferred from a welded fitting into a 200-mm steelchannel as illustrated in Fig. Estimate the maximum stress in the weld.

Example 2.3


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